Two-thirds of California voters believe global warming is a threat and measures need to be taken to stop it, but the level of concern has dropped significantly over the past six years, according to a Field Poll released Monday.

The poll found that 64 percent of Californians believe global warming is happening and something should be done to fight it, with more than half of the respondents, 37 percent, deeming it a serious problem worthy of immediate action.

The survey of 834 registered voters shows that Californians are still very much concerned about global warming, particularly after a series of unusual weather events over the past year, including flooding, Superstorm Sandy and record-level heat and drought across the middle of the country.

Skepticism is, nevertheless, higher than in the past. The percentage of climate-change believers is well below the 76 percent of people who felt the threat was worthy of action in 2007, according to the Field Poll.

Still, 70 percent of voters support California's landmark climate-change law, AB32, which implemented the first full-fledged cap-and-trade program and requires a reduction in carbon dioxide emissions by at least 17 percent over the next decade. The 2007 poll found that 79 percent of voters supported the law.

Mark DiCamillo, the director of the Field Poll, said concerns about climate change were higher in 2007 because former Gov. Arnold Schwarzenegger had just signed into law AB32, also known as the California Global Warming Solutions Act, amid great fanfare.

"There was this burst of enthusiasm about what the governor was doing," DiCamillo said. "This became his signature issue."

Concerns about climate change waned as the California economy tanked, hitting a low point in 2010 when polls showed a 20-point drop in support for AB32, he said. This was also around the time a well-funded national campaign to infuse doubt into climate science hit its stride.

The commonplace attacks on climate scientists, supported mostly by oil interests, have been widely debunked. Meanwhile, sea levels are rising and Arctic ice is melting.

The U.N. Intergovernmental Panel on Climate Change said sea levels have risen 6 inches over the past century and could rise more than 2 feet over the next 100 years if nothing is done to curb fossil-fuel emissions. Arctic sea ice shrank last summer to the smallest size it has ever been, covering less than half the area it did 30 years ago when satellites and submarines began measurements.

"An issue like global warming is kind of beholden to events, and this year we've had a lot of unusual weather events," DiCamillo said. "A lot of people are attributing these changes to global warming and that's probably why we've seen an increase in support."

The desire for action against climate change is highest in the Bay Area, with 77 percent of those surveyed there supporting measures to combat the problem, according to the poll. The Central Valley, with 51 percent supporting action, has the lowest percentage of support in California.

At least seven out of 10 Democrats, independents, voters under age 40, Asian Americans and college graduates support efforts to reduce carbon emissions. The least-supportive are Republicans, voters with only a high school education and folks living outside of the urban areas of Los Angeles and the Bay Area, according to the poll.

Voters of all stripes disapprove of the job the federal and state governments are doing on climate-change issues. Federal officials get the worst marks, with 62 percent disapproving of their efforts to deal with global warming. A plurality of 49 percent disapprove of the California government's efforts, with only 37 percent supporting.

Republicans are especially underwhelmed, with less than 20 percent approving of the job either the state or federal government is doing.

The Field Poll was conducted by telephone between Feb. 5 and 13. It has a sampling error of plus or minus 3.5 percentage points.